1. Field of the Invention
This invention relates to an insulating adhesive for use between circuit layers of a multilayer printed circuit board, and more particularly to an insulating adhesive of an epoxy resin type which is excellent in storage stability and can be rapidly cured at a high temperature of at least 100.degree. C., and which is preferably flame-retardant.
2. Description of the Related Art
In the production of a multilayer printed circuit board, there has been used a process which comprises putting at least one sheet of a prepreg obtained by impregnating a glass cloth substrate with an epoxy resin and semi-curing the resin, on an internal layer circuit board on which a circuit has been formed and further putting a copper foil on the prepreg sheet and-then molding the resulting assembly under pressure by a hot plate press. However in this process, the resin in the prepreg is allowed to flow again by heat and cured under a given pressure, and hence a period of 1 to 1.5 hours is required for uniformly curing and molding the prepreg. Since the production process requires such a long period of time and in addition a hot plate press and a glass cloth prepreg are necessary, the production cost becomes high. Moreover, since a method of impregnating a glass cloth with a resin is carried out, the thickness between the circuit layers is controlled by the thickness of the glass cloth, and hence, it has been difficult to make very thin the whole of the multilayer printed circuit board.
Recently, for solving the above problems, attention has been paid again to a technique of preparing a multilayer printed circuit board by a build-up system in which neither the heat-pressure molding by a hot plate press nor the glass cloth as an insulator between layers are used.
The present inventors have made extensive research on a process for producing a multilayer printed circuit board at a low cost by a simplified build-up system.
JP-A 7-202418 of the same as the present inventors discloses an insulating adhesive similar to the present one; however, no internal layer circuit board coated with an undercoating agent is used and hence the multilayer printed circuit board is insufficient in surface smoothness because of the difference in level between the insulating substrate and the circuit in the internal layer circuit board and many voids remain therein. Therefore, it has been difficult to put the multilayer printed circuit board to practical use.
After that, it has been generalized that in the production of a multilayer printed circuit board by a build-up system, when an insulating resin layer in the form of a film is used, the internal layer circuit board is coated with an undercoating agent for removing (or diminishing) the difference in level between insulating substrate and circuit in the internal layer circuit board to make its surface smoother. A typical example thereof is a method which comprises laminating a copper foil coated with an insulating adhesive to an internal layer circuit board coated with an undercoating agent in the state that the undercoating agent has not been cured or has been semi-cured or cured and curing the insulating adhesive to obtain a multilayer printed circuit board. By such a method, the difference in length between insulating substrate and circuit in an internal layer circuit board is made small, and hence, the lamination of a copper foil coated with an insulating adhesive is made easy and the necessity to consider the proportion of the copper foil residue in the internal layer circuit board becomes a little.
In such a process, there have been caused such problems that the insulating adhesive coated on the copper foil is too much softened during the lamination to ensure the insulation film thickness between the circuit layers; that the melt viscosity is too much lowered during the heat-curing and hence wrinkle is caused on the copper foil; that during the storage thereof, curing reaction proceeds and hence when the copper foil coated by the adhesive is laminated to the internal layer circuit board coated with the undercoating agent the integral molding is not well conducted; and that the adhesive layer is too hard to follow-up the irregularities of the internal layer circuit board coated with the undercoating agent and hence molding voids are caused. Furthermore, since no glass fiber substrate is used in the insulating adhesive between the circuit layers, there is such a problem that flame-retardation is difficult.
Such a technique is disclosed in JP-A 7-245480 of the same inventors as the present inventors; however, in this technique, no curing agent is contained in the undercoating agent coated on the internal layer circuit board, so that when an insulating adhesive (similar to the insulating adhesive of this invention) is laminated to the internal layer circuit board coated with the undercoating agent and then heated, the curing is not effected sufficiently and it has been impossible to put this technique to practical use.